\documentclass{article}
\usepackage{amssymb,amsmath,amscd,concmath}
\pagestyle{empty}
\begin{document}
\begin{equation*}
\begin{matrix}
\dot{x}&=&Ax+bu \\
\dot{\xi}&=&u  \\
y&=&cx+d\xi \quad (1) 
\end{matrix} \end{equation*}
\end{document}
